This invention relates in general to digitally encoded advanced television (ATV) including High Definition Television (HDTV) and particularly to ATV receivers that are operated in an area where NTSC co-channel signals and other interference exist.
In U.S. Pat. No. 5,260,793, a television receiver receives a digitally compressed ATV signal having a predetermined field sync code that is compared with a reference frame code to determine which of two paths should be used to process the signal. One of the processing paths subjects the received ATV signal, which is precoded in anticipation of NTSC co-channel interference, to a comb filter for reducing NTSC co-channel interference. The other path bypasses the comb filter. If the comb filtered field sync code yields fewer errors (i.e., is characterized by less interference or a higher S/N ratio) when compared with a reference frame code than the bypassed or direct path signal, the received ATV signal is subjected to NTSC co-channel interference reduction comb filtering. If the comparison of the combed field sync and reference codes does not produce significantly fewer errors than the comparison for the direct signal path, no comb filtering is used since the comb filter introduces a 3 dB increase in white noise. Most ATV receivers will also include adaptive channel equalizers to assist in adjusting the receiver response to optimize reception of the entire signal in response to a known equalizing (training) signal.
It is often very difficult to differentiate between cochannel interference and other types of interference. This makes the decision of whether to subject the received ATV signal to the NTSC co-channel interference rejection comb filter very difficult. For example, certain recurrent interference, such as slight DC offsets and other types of static linear distortion, can easily mislead the circuitry responsible for determining the comb filter mode. There is also difficulty associated with intersymbol interference in the ATV multi level signal which results in signal-to-noise degradation even without co-channel interference. Other interference such as "ghosts" add to the difficulty of determining the NTSC co-channel filtering mode. With the present invention, more accurate systems for determining the correct operating mode for the NTSC co-channel interference reduction system is provided.